1. Field of Endeavor
The present invention relates to a lasers, and more particularly to a laser system that provides high power and high beam quality.
2. State of Technology
There are many applications for very high average optical power lasers at near infrared, or shorter wavelengths. These applications include power beaming, laser guide stars, illuminators, material processing, and weapons. In power beaming, laser radiation is transported to a remote location where it is converted to electrical power or used for other uses such as propulsion. Tens to hundreds of kilowatts of laser light with sufficient beam quality are required for the missions of interest. The wavelength requirements of the receiver as well as the transparency of the atmosphere are prime considerations.
A problem in high power applications that has been encountered is the lack of suitable sources of radiation with high beam quality. For weapons applications the Chemical Oxygen Iodine Laser (COIL) seems to show promise, but it is a complex and expensive system for use in the 10-100 kW regime. Also at 1.3 micron, it is not useful for use with solar cells and is not readily frequency converted to short wavelengths as would be required for guide stars.
Free electron lasers (FELs) have been touted as a solution for decades, but have failed to demonstrate useful output powers and acceptable system efficiencies at the wavelengths of interest. Proposed high efficiency recirculating FELs, in which the beam energy is recovered after exiting the wiggler, have been unimpressive due to the poor emittance of the recovered beam and the resulting energy loss in the accelerator and storage ring. To date FELs have yet to produce multi-kilo watt beams at any wavelength and require an immense physical plant to produce even a few watts in the infrared.
Neodymium and other metal ions embedded in a variety of glassy and crystal hosts have been the mainstay of commercial lasers and fusion lasers for decades. However, when these devices are applied to multi-kilowatt applications results have been disappointing. Despite of the development of many complex and expensive systems, practical operation at much greater than one kilowatt with beams near the diffraction limit has not been demonstrated. This complex problem is essentially an issue of thermal management. Solids must have the heat conducted away and at high heat loads this leads to unavoidable temperature gradients. These gradients destroy optical beam quality and can lead to fracture of the media. To cope with these problems designs have recently been put forward that use reduced crystal thickness to improve heat conduction. This approach has limited excited volume and a complex optical train must be constructed to give the volume needed to produce high average powers. It is not clear if closure can be reached with these designs since each added component adds additional optical aberrations and opportunities for catastrophic optical damage.
Fluids, on the other hand, have the advantage that when the media gets hot it can be removed from the optical path to a place where it can be cooled. A liquid laser is shown and described in U.S. Pat. No. 3,717,825 to Carl Zeiss-Stiftung, Wuerttemburg, Federal Republic of Germany, patented Feb. 20, 1973. This patent shows a dyestuff laser provided with a liquid guiding chamber through which circulates a cooled laser liquid. The laser is provided with a U-shaped laser active zone formed by a light transmitting longitudinal cap into which extends a tongue forming in said cap a U-shaped zone of uniform cross section. This U-shaped laser active zone is disposed in a focal line of an elliptically shaped pump light reflector while a source of pump light is disposed in the other focal line.
Fluids do not exhibit birefringence so all polarization options are available, have high optical damage thresholds and do not permanently damage if the threshold is exceeded. Compared to solid laser media fluids are quite inexpensive. U.S. Pat. No. 3,931,594 to Fritz Peter Schafer, assigned to Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V., patented Jan. 6, 1976, shows a transverse-flow cell for a laser. The cell of a liquid laser is defined between the rounded, parallel edges four coaxial cylinder sectors of fused quartz. The narrow gaps circumferentially bounded by the flat, converging side faces of the sectors provide conduits for rapid flow of dye liquid into and out of the cell which extend over the full axial length of the cell. The light of a flash tube is directed toward the cell through the cylindrically arcuate outer face of each sector.
U.S. Pat. No. 3,740,665 to Irving Itzkan, assigned to Avco Corporation, patented Jun. 19, 1973 shows a transverse flowing liquid laser. Stimulated emission of radiation (laser action) is produced in materials generally classed as dyes. These dyes are dissolved in a liquid solution. A quantity of dye in a flowing liquid solution in a module or cavity is pumped or excited by a laser beam radiating in the ultraviolet region which is focused to a line with a cylindrical lens. A rectangular cross sectional beam of such radiation is produced by a pulsed crossed field nitrogen gas laser. The focused line which is transverse to the beam produced by the exciting laser, and transverse to the direction of the flowing dye lies near the surface of the dye material in the cell, and is substantially as long as the cell is wide. The cell lies within an intensifying optical cavity which may be formed by a 100 percent reflecting mirror and a partially reflecting mirror both perpendicular to the line of focus of the pumping radiation. The stimulated emission from the dye material is characterized by a short pulse width and a little loss of energy between the two lasers.
U.S. Pat. No. 3,678,410 to Robert C. Kocher, Franklin K. Moore, Harold Samelson, and William R. Watson, assigned to GTE Laboratories Incorporated, patented Jul. 18, 1972 shows a transverse flowing liquid laser. A laser cell for a transverse flow liquid laser has an active region in the form of a rectangular prism and cylindrical input and output chambers mounted in spaced-apart relationship in the transverse direction at opposite ends of the active region. A baffle positioned in the input chamber causes the liquid to flow uniformly through the active region.
One of the most highly developed fluid lasers is the AVLIS dye laser AVLIS (Atomic Vapor Laser Isotope Separation). Development of this family of lasers took place at Lawrence Livermore National Laboratory over the period of 1972 to 1999. It can produce single aperture powers approaching 3 kW with nearly diffraction limited beams. However, these lasers do not store optical energy and must be excited by an even higher peak power pump laser, albeit with much lower beam quality requirements. Rare earth ion based lasers have the advantage of long florescent lifetimes so they require lower peak pump powers. The present invention uses a fluid host for a rare earth ion to get the advantages of a fluid that stores optical energy.
Liquid hosts containing rare earth have been considered. However, such devices were flash lamp driven leading to unacceptably large temperature gradients in the fluid and their poor beam quality. U.S. Pat. No. 3,931,594 to Erhard J. Schmitschek, assigned The United States of America as represented by the Secretary of the Navy, patented Dec. 18, 1973, shows a liquid lasing composition consisting essentially of neodymium (III) phosphorus dichloridate, retained in solution with phosphorus oxychloride by the addition of a Lewis acid. A liquid lasing solution is prepared by introducing neodymium trifluoroacetate into phosphorus oxychloride. Some Lewis acid is also desirably included in the solution in an amount increase solubility of the neodymium III phosphorus formed, optimize the intensity of fluorescense of the liquid solution and enhance its operative efficiency. Preferably, Lewis of the form zirconium tetrachloride is employed and the neodymium (III) phosphorus dichloridate formed as a result of the dissolution of the neodymium trifluoroacetate in the phosphorus oxychloride is in a mole proportionality relative to the zirconium tetrachloride of not less than 1. The liquid laser solution is highly stable over relatively long periods of time, does not degrade under flash excitation, and obviates certain undesirable absorption characteristics of comparable prior art liquid lasing compositions.
It is an object of the present invention to provide a laser system that will produce high optical power with high beam quality. A semiconductor pumping device is used to optically excite a liquid laser. Improved thermal efficiency and high power dye laser features provide a system that has the optical storage advantage of the solid laser with the high average power performance of a dye laser. An aspect of the present invention is to provide a laser system that utilizes features of aprotic host lasers, semiconductor diode lasers, and dye lasers. Another aspect of the present invention is to provide a high power laser device using rare earth salts dissolved into aprotic liquids optically excited by semiconductor diode laser. Consideration must be give to the nature of the liquid host. Manufacture, operation, and ultimate disposal of the laser and its components becomes expensive and difficult if the liquid host contains highly hazardous materials. An aspect of the present invention is to provide a new laser system that has the optical storage advantage of a solid laser with the high average power performance of a dye laser. The laser system does not use highly hazardous materials and provides high power and high beam quality.
In an embodiment of the present invention a flowing non-hydrogen containing (aprotic) liquid, containing rare earth ions (neodymium for example) is optically excited by semiconductor diode lasers resulting in a powerful near infrared laser similar in properties to a solid state laser using a glass or crystal host. Since the host is a liquid, it can be removed from the optical cavity when it becomes heated avoiding the inevitable optical distortion and birefringence common to glass and crystal hosts. Also because rare earth ionic concentrations in aprotic liquids can be significantly higher than those in glass and crystal hosts, better energy defect optimization can be made by using weaker pump absorption bands closer in energy to the laser emissions. This helps to further reduce the heat induced distortion to the laser medium. The aprotic liquids are far less hazardous than the laser liquids of the prior art.
Additional aspects, advantages, and features of the invention are set forth in part in the following description. Various aspects, advantages, and features of the invention will become apparent to those skilled in the art upon examination of the description and by practice of the invention.